Fabry-Perot interferometers are used as optical filters and in spectroscopic sensors, for example. The Fabry-Perot interferometer is based on parallel beam splitter mirrors, whereby a Fabry-Perot cavity is formed into a gap between the mirrors. The pass band wavelength of a Fabry-Perot interferometer can be controlled by adjusting the distance between the mirrors i.e. the width of the gap. It is common to use micromechanical technology for producing Fabry-Perot interferometers because of low production costs of the technology. However, there are some limitations related to the micromechanical technology. The adjustment range of the mirror position is generally low, corresponding to +/−20-25% adjustment range of the wavelength of the interferometer. The maximum adjustment frequency of the mirror is also low, approximately 200-500 Hz. Fabry-Perot interferometers are also commonly made with liquid crystal technology, but this technology has similar deficiencies. Therefore, in many applications it is preferable to use Fabry-Perot interferometers which are based on using controllable actuators between mirrors, such as piezoelectric, electrostrictive or flexoelectric actuators.
FIG. 1 illustrates a prior art Fabry-Perot interferometer 10. The interferometer includes two mirrors 11 and 12, which have a substrate is of transparent material, and the surfaces between the mirrors have a thin metal or dielectric coating 13, 16 in order to provide partial reflection of radiation. Two, three or four actuators 14 are attached with glue at their opposite sides 17 between the mirrors 11, 12. The pass band wavelength of the interferometer is adjusted by applying voltage to the actuators 14. The dimensions of the actuator are controlled by the applied voltage, and thus it is possible to control the distance between the mirrors, i.e. the width of the gap/cavity.
Some disadvantages are related to the prior known Fabry-Perot interferometers in which controllable actuators are used. The electrodes of the prior art Fabry-Perot interferometers are made by coating a metal layer on the mirror substrate. If the interferometer has a very small gap, the electrodes may easily touch each other. In order to avoid the electrodes from touching each other, it is necessary to use a sufficiently wide gap between the mirrors. In other words, it is not possible to provide interferometers with very small gaps.
There is another problem related to achieving a uniform gap between the mirrors. When mirrors are glued to the actuators the glue has a property to shrink during the hardening of the glue. The shrinkage of the glue tends to cause bending of the mirrors, whereby the size of the gap between the mirrors becomes non-uniform. When the gap is not uniform with a required accuracy, this further causes the functional wavelength band of the Fabry-Perot interferometer to become wider and shifted. The bending also increases the risk of the mirrors touching each other, and to avoid such touching of mirrors it is necessary to use a larger average gap between the mirrors.
In order to alleviate the problem due to shrinking of glue, the prior art mirrors are made of thick material in which bending is small. However, the use of thick mirror substrates causes the weight and manufacturing costs of the Fabry-Perot interferometer to become high. The thick mirror substrate also tends to attenuate radiation. This may, for example, cause decreasing of signal-to-noise ratio when a Fabry-Perot interferometer is used in measuring radiation with low intensity.